The present invention relates to a method of and apparatus for effecting development by use of a one-component type developer in electrophotography and electrostatic recording.
The conventional developers for use in electrophotography and electrostatic recording can be roughly classified into a two-component type developer comprising toner and carriers and a one-component type developer consisting of only toner. The former developer can provide excellent copy images, while it needs a complicated detection apparatus for maintaining the toner concentration of the developer. Furthermore, in the two-component type developer, the carrier is not consumed and is used repeatedly, so that fatigue of the carrier takes place, which brings about toner deposition on the background of copies during a long use of the developer and the detection accuracy of a toner concentration detection apparatus is lowered while in use. On the other hand, the latter developer consists of only toner. Accordingly, it does not have the above-mentioned shortcomings and it is suitable for use in inexpensive copiers and the copiers using the one-component type developer rarely need maintenance.
However, the development method using the conventional one-component type developer is not always flawless, but has various shortcomings. For example, in a pressure application development method disclosed in Japanese Pat. No. Sho-52-36414, toner is deposited on a toner support member which is lined with a conductive and elastic foam material and the toner is charged to an opposite polarity to that of a latent electrostatic image by a plurality of triboelectric charging members during transporation of the toner and the toner support member is brought into pressure contact with a photoconductor and the toner support member and the photoconductor are moved in the manner that their relative peripheral speeds are substantially zero, whereby the latent electrostatic image is developed. In this method, a bias voltage is applied to the toner support member at the time of development in order to prevent toner from being deposited on the non-image area of the photoconductor. However, in the case where the original image is low in the contrast, development cannot be performed sufficiently. In other words, in the case where the original image is low in the contrast, consisting of light-colored letters and white background, application of a bias voltage for preventing deposition of toner on the background will cause the letter portion to disappear since there is little difference in the potential between the image area and the non-image area. Thus, the image area cannot be reproduced. Furthermore, in the case where the original image is low in the contrast, consisting of a dark background and darker letters, the non-image area is at a high potential. Accordingly, application of a high bias voltage for preventing deposition of toner on the non-image area will cause the leakage of the bias current from the toner support member to the photoconductor since the photoconductor and the toner support member are in pressure contact with each other. This will damage the photoconductor.
In Japanese laid-open patent applicaton No. Sho-50-117432, there is disclosed a method for charging toner on a development roller by a voltage applied electrode. Furthermore, in a development apparatus described in Japanese laid-open patent application No. Sho-52-81228, a toner layer as thin as 20 to 60 .mu.m is formed on a development roller and a doctor blade which serves an an electrode is brought into pressure contact with the development roller by spring means. The toner layer formed on the development roller is narrower than the development roller in the axial direction thereof and also narrower than the lengthwise width of the above-mentioned electrode. Accordingly, the opposite end portions of the electrode may contact with the opposite portions of the development roller so long as there is no toner therebetween, so that current is apt to leak from the electrode to the development roller. When current leaks from the electrode to the development roller, the potential difference between the two becomes small and the charging efficiency of toner is lowered. This could be prevented by making the width of the electrode equal to that of the toner layer. However, this is extremely difficult in practice. For instance, when the electrode is narrower than the toner layer, the toner which is not in contact with the electrode remains uncharged and accordingly, toner cannot be used sufficiently for development. On the other hand, when the electrode is wider than the toner layer, there is a danger that the above-mentioned current leakage may take place.